staging: rtl8723bs: convert private allocation functions to return void *

Using char * for a return from allocation functions means the
code has to cast generic allocations to specific types.

Allow the compiler to not need casts on the allocations.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>